Low-cost multi-laminate sensors

ABSTRACT

Sensors for monitoring environmental variables such as chemical composition are disclosed. These low-cost sensors comprise multiple layers in a laminated stack. Very high numbers of sensing layers (e.g., 500) may be incorporated into a single laminated sensor device. The sensors may be produced from nanostructured materials. Additionally, multilayer magnetic, optical, photonic, thermal and biomedical sensors are disclosed.

This application is a divisional of U.S. patent application Ser. No.09/074,534 titled “LOW-COST MULTI-LAMINATE SENSORS” filed May 7, 1998,now U.S. Pat. No. 6,202,471, which claims the benefit of priority toU.S. Provisional Patent Application No. 60/061,718, filed Oct. 10, 1997,and No. 60/068,121, filed Dec. 19, 1997.

BACKGROUND OF THE INVENTION

The ability to detect the presence and composition of chemical specieshas been an important goal for several reasons. For example, thedetrimental environmental effects of toxic species such as formaldehyde,carbon monoxide, ozone, hydrocarbons, chlorocarbons, nitrogen oxides,aromatics and heavy metals has led to the need to develop efficient,sensitive, and affordable ways of detecting the composition and presenceof such toxic substances. Additionally, the efficiency of chemicalprocesses, in terms of energy and raw material used per unit product orservice delivered, relies on the ability of the overall system toreliably sense deviations from the optimal processing conditions. Sinceprocess efficiencies directly determine the overall costs of the processand indirectly determine the wastes generated by the process, it iscritically important that a method be available that can provide thenecessary feedback about the process (sensors) and initiate actions toevolve the system parameters to the optimal levels (actuators).

The temperature, pressure and flow monitoring and control of chemical,environmental, biochemical, biomedical, geological, metallurgical, andphysical processes have been extensively researched and thestate-of-the-art technologies quite effectively enable real-timeevolution of the monitored process. However, compositional monitoringand control of these processes leaves much to be desired. Crude methodsfor process monitoring and control are based upon batch analysis, i.e.,a statistical set of samples are obtained (“grab sample” approach) andthen analyzed for composition. These data are then interpreted andactions are initiated to control the process to desired levels. Theresponse time for such a strategy often is in days, if not weeks. Thisstrategy has serious deficiencies since it inherently acceptsinefficient operation between the time the samples were obtained and theactions are initiated to correct deviations from the optimal. Yetanother deficiency in such a strategy is that it overlooks thepossibility that the process conditions may have changed during theresponse and analysis lag time.

Alternatively, sophisticated monitors (such as gas chromatographs withsuitable sampling and transport systems) have been integrated into theprocesses. These systems are expensive, bulky, not suited for extremetemperatures and pressures, and have response times that are more than afew minutes. Real-time composition monitoring and control of thechemical and combustion processes require sensors that overcome theselimitations. Specifically, sensor technology for gas sensingapplications should ideally be selective, sensitive to trace species,fast (short response time), small, accurate, reproducible, stable inextreme environments, durable (long life), and affordable.

Prior art methods for producing gas sensors include those of U.S. Pat.No. 4,631,952 which teaches a method of preparing a sensor by theformation of a dispersion of conducting particles with a materialcapable of swelling in the presence of the liquid, gas or vapor to besensed. Furthermore, U.S. Pat. No. 4,984,446 teaches the preparation ofa gas detecting device by a layer by layer build up process, and U.S.Pat. No. 4,988,539 teaches an evaporation-deposition method and processfor manufacturing a gas detection sensor. Finally, U.S. Pat. No.5,387,462 teaches a method of making a sensor for gas, vapor, and liquidfrom a composite article with an electrically conductive surface havingan array of whisker-like microstructures with an aspect ratio of3:1 to100:1.

Although these prior methods provide improved methods for producingsensors, there is still a need to develop sensors which are selective,sensitive to trace species, fast, small, accurate, reproducible, stablein extreme environments, durable, and finally affordable.

SUMMARY OF THE INVENTION

In one aspect, the present invention involves a sensor device comprisinga laminated structure including multiple sensing and/or electrodelayers, each of which may be of the same or different compositions. Thestructure may have 3-500 layers, or in other embodiments 10-100 layers,or in still other embodiments 20-50 layers. The slices may be calcinedand sintered before they are cut. The sensor may be partially orcompletely coated, for example to protect the electrodes fromenvironmental damage. The sensing layers may be prepared using powdersor composites, e.g., nanostructured powders and nanocomposites. Theinteraction between the sensor and the analyte may be physical,chemical, electronic, electrical, magnetic, structural, thermal,optical, surface, or some combination. The sensor may include layersother than the sensing and electrode layers, for example, heating orinsulating layers.

Definitions

“Sensitivity,” as that term is used herein, is a dimensionless measureequal to the ratio of the change in a measured property to the originalvalue of that property. For example, the sensitivity of a chemicalsensor whose resistance is a function of chemical environment is definedas ((R_(a)−R_(s))/R_(a)) where R_(a) represents the resistance of thesensor in the absence of the sensed chemical species, and R_(s)represents the resistance of the sensor in the presence of the sensedchemical species.

DESCRIPTION OF THE DRAWING

The invention is described with reference to the several figures of thedrawing, in which:

FIG. 1 depicts a diagram of a laminated multi-layer sensor according tothe invention;

FIG. 2 depicts the process of terminating and cutting a laminated stackto produce the sensor shown in FIG. 1;

FIG. 3 is a circuit diagram illustrating the use of the sensor depictedin FIG. 1;

FIG. 4 shows an illustrative electrode pattern for the sensors of theinvention;

FIG. 5 is a backscatter image of the sensor of Example 1;

FIG. 6 is an SEM micrograph of the sensor of Example 1;

FIG. 7 presents the response data of the sensor of Example 1 to 5%hydrogen gas; and

FIG. 8 presents the response data of the sensor of Example 1 to watervapor.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

In one aspect, the present invention provides sensing devices and, morespecifically, chemical composition sensors and a fluid compositionsensors. Additionally, a method for the use of a chemical compositionsensor to detect the composition of a species of particular interest isdisclosed in the presently claimed invention. In another aspect, thepresent invention provides novel fluid and chemical composition sensorsfor detecting the presence and composition of liquids, vapors and gases.

According to the presently claimed invention, the method for thesynthesis of a chemical composition sensor or a fluid composition sensorinvolves selecting a particular species of interest and then selecting aparticular material composition that is capable of interacting with thespecies of interest. If the selected material composition is capable ofinteracting simultaneously with multiple species of interest,compositions that interact equally with two or more species arepreferred for the practice of the invention. In a preferred embodiment,the material compositions interact with one species. In anotherpreferred embodiment, the material composition may interact with up to(n−1) species where n is the number of species. In yet another preferredembodiment, the material composition may interact with all n species onan unequal basis. In all cases, this invention requires that thematerial compositions chosen be physically robust in presence of allspecies in its environment for a duration equal to or greater than thedesired life for the sensor. It is also desirable that more than n suchmaterial compositions are identified and used, because, in the eventthat one element fails, the redundant elements provide the necessarysignal for reliable operation of the sensor.

The interaction of the species with the material composition in thisinvention necessarily requires that the material's properties detectablychange when the species and material composition are exposed to eachother materially, mechanically, thermally, optically, orelectromagnetically, or a combination of these. Any property changewhich can be reliably detected with a satisfactory signal-to-noise ratiomay be used, including physical changes, chemical changes, electricaland electronic changes, magnetic changes, structural changes, opticalchanges, thermal changes, surface changes, or any combination of these.In preferred embodiments, physical interactions include but are notlimited to adsorption, absorption, expansion, shrinkage, and phasetransformation. In other preferred embodiments, chemical interactionsinclude but are not limited to reaction with the material,isomerization, dissociation, and polymerization. In yet other preferredembodiments, electrical and electronic interactions include but are notlimited to changes in dielectric constants, charge transfer, and holetransfer.

Once a suitable material composition has been selected for use in thechemical composition sensor, in one embodiment, namely the formation ofa chemical composition sensor, a laminated stack structure 10 (FIG. 1),preferably multi-layer, is formed using the identified sensing material.The laminated stack contains at least one sensing layer 12, and at leastone electrode layer 14. The sensor shown in FIG. 1 also compriseselectrical terminals 16, in electrical communication with the electrodelayers 14.

The sensing layer 12 can be formed by any method, including but notlimited to spin coating, dip coating, powder pressing, tape casting,screen printing, curtain deposition, physical sputtering, reactivesputtering, physical vapor deposition, chemical vapor deposition, ionbeam, e-beam deposition, molecular beam epitaxy, laser deposition,plasma deposition, electrophoretic deposition, magnetophoreticdeposition, 25 thermophoretic deposition, stamping, centrifugal casting,gel casting, extrusion, electrochemical deposition, screen-stencilprinting, brush painting, or a combination of one or more of theabove-mentioned methods. The sensing layer thickness can be in the rangeof 1 nm to 10 cm, a preferred thickness being less than 100 microns, amore preferred thickness being less than 10 microns, and a mostpreferred thickness being less than 1 micron. In a preferred embodiment,the sensing layer can also be doped with catalysts to further enhancethe selectivity of interaction or the kinetics of interaction, or dopedwith promoters to enhance the ability to detect the changes in theproperties of the material composition. These secondary treatments ofthe sensing layer may enhance the performance of the sensing layer.

Possible compositions of the sensing layer include but are not limitedto organic, inorganic, metallic, alloy, ceramic, conducting polymer,non-conducting polymer, ion conducting, non-metallic, ceramic-ceramiccomposite, ceramic-polymer 10 composite, ceramic-metal composite,metal-polymer composite, polymer-polymer composite, metal-metalcomposite, processed materials including paper and fibers, and naturalmaterials such as mica, bio-organisms, biologically active materials,biologically derived materials, biocomposites, or a combination of oneor more of these. Illustrative compositions include but are not limitedto doped or un-doped, stoichiometric or non-stoichiometric titaniumoxide, barium titanate, strontium titanate, zinc oxide, indium oxide,zirconium oxide, tin oxide, antimony oxide, tungsten oxide, molybdenumoxide, tantalum oxide, cerium oxide, rare earth oxides, silicon carbide,bismuth telluride, gallium nitride, silicon, germanium, iron oxide,titanium boride, iron boride, zirconates, aluminates, tungstates,carbides, borates, hydrides, oxides, oxynitrides, oxycarbides, halides,silicates, phosphides, nitrides, chalcogenides, complex oxides such asdielectrics and ferrites, enzymes, nucleotides, antibodies, cells, andpolymers.

Additionally, the sensing layer can be porous or dense, flat or tapered,uniform or non-uniform, planar or wavy, straight or curved,non-patterned or patterned, micron or sub-micron, grain sized confinedor not, or a combination of one or more of these.

In preferred embodiments, the electrode layer 14, as shown in FIG. 1,may comprise any composition that conducts the signal more easily thanthe sensing layer, most preferably with a lower impedance than thesensing layer. If the signal is electrical signal, the electrode layeris any composition that conducts electricity more easily than thesensing layer, preferably with a lower impedance than the sensing layer.It is most preferred that the electrode impedance be very low,preferably below 10% of the sensing material layer's impedance. In caseproperties other than electrical are utilized for sensing, the impedanceto the signal should be less in the electrode layer than the sensinglayer. If the impedance of the electrode is high, then it is preferablethat the electrode impedance be predictable and that the metrologycompensate for the electrode impedance to achieve satisfactory signal tonoise ratio. The composition of the electrode layer can include, but isnot limited to, organic materials, inorganic materials, metallic, alloy,ceramic, polymer, non-metallic, ceramic-ceramic composite,ceramic-polymer composite, ceramic-metal composite, metal-polymercomposite, polymer-polymer composite, metal-metal composite, or acombination of one or more of these. Geometries may be porous or dense,flat or tapered, uniform or non-uniform, planar or wavy, straight orcurved, non-patterned or patterned, micron or sub-micron, grain sizeconfined or not, or a combination of one or more of these. Methods offorming the electrode layer include but are not limited to spin coating,dip coating, powder pressing, tape casting, screen printing, curtaindeposition, physical sputtering, reactive sputtering, physical vapordeposition, chemical vapor deposition, ion beam, e-beam deposition,molecular beam epitaxy, laser deposition, plasma deposition,electrophoretic deposition, magnetophoretic deposition, thermophoreticdeposition, stamping, centrifugal casting, gel casting, extrusion,electrochemical deposition, screen and stencil printing, brush painting,or a combination of one or more of such methods.

As shown in FIG. 1, the sensor is comprised of laminated layers ofsensing layers 12 and electrodes 14. The sensor may just have one layeror multiple layers, the preferred embodiment being multiple layers. Theindividual sensing layers or electrodes can be the same or differentformulation. Additionally, it is possible to replace or combine one ofthe sensing layers with a layer capable of a secondary but desiredfunction. For example, the sensing layer can be replaced with resistivelayers by design to provide heat to the sensor. In some situations itmay be desirable to have one or more sensing layers replaced with EMI(electromagnetic interference) filter layers to minimize noise byinductively or capacitively coupling with the sensing layer. In anothersituation, one of the layers can be air or an insulating layer in orderto provide thermal isolation to the active layer. In yet anothersituation, electrochemical couple layers may be provided to internallygenerated electricity and energy needed to satisfactorily operate thesensor. In other conditions, the electrode layers can function as anodesand cathodes. In some situations, the sensor may be a minor part of themulti-laminate device and the sensor containing device can have primaryfunction of reliably providing an electrical, thermal, magnetic,electromagnetic, optical, or structural function in an application. Thesensing layer can also comprise multi-laminates of different materialformulations.

The multi-layer stack 10, as shown in FIG. 1, has a rectangular shape.However, the stack shape can also be circular, elliptical or any othershape. Additionally, the edges may be rounded or sharp. The electrodesin the multi-layer stack can be connected to yield a single sensorelement structure or connected selectively to yield a sensor array.Furthermore, in case of arrays, common electrodes for two or moresensors may be used to reduce the external lead costs and crowding. Thetermination can similarly be internal or external to satisfy theparticular needs of an application.

After preparation of a multi-layer stack, the stack may then be cutcross sectionally into thin slices to expose the alternating layers ofthe sensing layer and the electrode. FIG. 2a shows a terminated stack10, which is saw cut along lines A—A, B—B, and C—C as shown in FIG. 2b.FIG. 2c shows the exposed alternating layers at the surfaces of theslices 18. Each slice is a device which can be used in a circuit shownas FIG. 3 to detect the composition of an analyte in a gas, vapor,liquid or a combination of these. The circuit shown in FIG. 3 isillustrative; it may be replaced with any suitable circuit that candetect the signal to noise ratio acceptable in a given application.

The multi-layer stack can be a single element or an array of sensingelements. In the case of a single element body, single element sensorsresult after the slicing operation. In the case of an array elementbody, an array of sensor elements result after the slicing operation.After the slicing operation, the exposed cut surfaces provide thesensing surfaces, while the unexposed surfaces isolate the device. Theelectrodes of the device provide means to connect the sensor to anexternal circuit and obtain the signal.

In a preferred embodiment, it is desirable that the sensing layers andthe electrode layers be isolated from external environmental damage suchas that from thermal, mechanical, chemical, electrical, magnetic, orradiation effects, or a combination of one or more of these. Thisdesired protection may be achieved by providing a conformal coveringover the layers on the unexposed surfaces, such as an polymer conformalprotective layer. In another preferred embodiment, the exposed surfacemay also be isolated from external thermal, mechanical, chemical,electrical, magnetic, or radiation damage by covering with a layer ofceramic or porous rigid material mesh. In yet another preferredembodiment, the exposed surface may also be covered with a layer thatenhances the selectivity of the analyte reaching the sensing surface.Such a layer can include, but is not limited to, polymers, metals,zeolites, or porous media, each of which has a higher permeability forthe analyte of interest and a lower permeability for other species thatare not of interest. In some preferred embodiments the exposed surfaceis covered with polyethylene, polypropylene, Teflon, polycarbonates, orpolyaromatics. However, it is generally preferable that any covering onthe exposed surface does not impede the interaction of the analyte oranalytes to be sensed with the active sensing layer by an amount greaterthan the species that are not of interest. Exceptions to this generalrule maybe made in certain cases, for example, when it is critical toprotect the element from destructive effects of the environment.

In a preferred embodiment of the present invention, the sliced sensorelements are also calibrated. The calibration is preferably performedwith a gas, vapor or liquid mixture whose composition of one of thedesired species is varied. During the calibration, the impedance valueversus the concentration of the varying species is obtained. Suchcalibration data is preferably obtained for all species of interest. Inthe event that only complex data is available, pattern matching software(e.g., neural networks) can be utilized to correlate the response of thedevice to each specific analyte.

In another particularly preferred embodiment, a method for manufacturinga fluid composition sensor is provided. This method involves selecting achemical species of interest, selecting a material composition thatselectively interacts with the species of interest, preparing a quantumconfined or nanometer cluster form of the selected sensing material,preparing thin monolithic or composite film arrays of quantum confinedor nanocluster forms of sensing materials, and finally connecting leadsto form the sensor array.

The selection of the chemical species of interest and the selection ofthe material composition have previously been discussed with regard tothe manufacture of the chemical composition sensor. Subsequently,quantum confined and nanometer cluster forms of the identified materialcompositions are prepared. In preferred embodiments, this can beaccomplished by chemical precipitation techniques, vapor condensation,thermal quench, pyrolysis, mechanical alloying, explosive or chemicaldisintegration, laser beam evaporation, template synthesis, or acombination of these methods. Once the quantum confined and nanometercluster form of the material compositions are available that interactwith the species of interest, laminated thin films of the sensingmaterials are prepared on electroded substrates. The electrodes areprovided to enable the measurement of impedance changes in the film withcomposition changes. In particularly preferred embodiments, theelectrodes are prepared by screen printing, tape casting, spraying,sputtering, chemical vapor deposition, physical vapor deposition,epitaxy, electrochemical deposition, mold stamping, and cutting througha multilayer to expose the electrodes. It is particularly preferred thatthe electrodes be uniform, without sharp edges, and reproducible betweenthe sensors. FIG. 4 shows two views of an illustrative electrodepattern. The electrodes 14 in this pattern can be exposed fortermination by cutting at lines A-A′ and B-B′. For selectivity, in apreferred embodiment, one may integrate a resistive element in thesubstrate to heat the sensor to a desirable temperature to facilitatethe interaction of the species of interest to the material compositionas desired. However, if the heating element is integrated, care must betaken to avoid shorting of the electrode or transmission of noise fromresistive element to impedance signal from the electrode. This can beaccomplished with the use of capacitive, resistive, or inductive film,or a combination thereof.

Once the electroded substrate is available, the quantum confined andnanometer cluster form of the material is formed into thin or thickfilms preferably using, but not limited to, one of the followingmanufacturing methods or a combination thereof including masked screenprinting, masked tape casting, masked spraying, masked sputtering ofnanostructured targets, masked vapor deposition, masked epitaxy, maskedelectrochemical or electrophoretic deposition of cluster, and maskedmold stamping. In a most preferred embodiment, a single manufacturingmethod is used to prepare all sensing elements in a sensor. Since thepresently claimed invention requires that elements of different materialcompositions be deposited to detect different species of interest, it isinherent to change the mask and repeat the element forming step.

After the step of forming the films, the film is preferably dried andsintered at low temperatures to ensure mechanical robustness. The use oflow temperatures is preferred because high temperatures can cause graingrowth and consequent loss of surface area and confinement-relatedproperties. It is particularly preferred that the sintering temperaturesbe less than 0.75T_(m), where T_(m) is the melting point of the film'smaterial. If necessary, higher temperatures can be used, however it isthen preferred that the sintering time be minimized.

Once the film on the electrode has been deposited, the leads areconnected to the electrodes. To minimize the costs, it is preferred thata common electrode be used. The common electrode is one that connectsall the sensor elements. The second free electrode of each element isnext used to determine the impedance of each element as a function ofthe sensor element's environment.

Finally, after the sensor elements have been produced, they arecalibrated. The calibration is preferably performed with a gas mixturewhose composition varies only in one of the desired species. Theimpedance value versus the concentration of the varying species isobtained. Such calibration data is then obtained for all species ofinterest. In the event that only complex data is available, patternmatching software such as neural networks can be employed.

The novel chemical composition sensors and fluid composition sensors asdescribed can be utilized to determine the composition and presence ofvapors, gases and liquid analytes. The sensor element or array can beconnected in a circuit to prepare a monitor capable of detecting changesin the property of the sensing layer. The sensor may be used as adisposable sensor or as a non-disposable sensor, as a regenerable ornon-regenerable sensor, or as a continuous or cumulative sensor. In aparticularly preferred method, the sensor element or sensor elementarray is first exposed to the environment whose composition needs to bemeasured. The sensing mechanism can be based, for example, onresistance, capacitance, inductance, impedance, phase angle, lossfactor, dissipation, breakdown voltage, electrical temperaturecoefficient of an electrical property, Nernst current, impedanceassociated with ion conducting, open circuit potential, electrochemicalproperty, electronic property, magnetic property, thermal property,mechanical property, or optical property. In a particularly preferredembodiment, the impedance value is obtained and interpreted in light ofthe calibration data. The interpreted impedance then yields the desiredcomposition information. One of ordinary skill in the art willappreciate that particular sensor designs and sensing layer propertiesare specifically designed to detect particular compositions. Therefore,the choice of particular sensor designs and sensing layer properties maylimit the potential use of the sensor elements or arrays.

The examples presented below are intended to more particularly describethe present invention, but are not intended to limit the scope of thepresently claimed invention.

EXAMPLES Example 1

Multi-layer ZnO Sensor

Multi-layer Pt/ZnO/Pt structures of initial size 0.125″×0.100″×0.058″(L×W×H) having 12 layers of ZnO were lapped and polished to expose theactive multi-layered sensor surface as follows: After mounting thesamples on the polishing surface with a crystal bond wax, they were thenpolished to 600 grit on both sides. The final components shown in thebackscatter images (FIG. 5) had dimensions of 0.125×0.100×0.029 (L×W×H),and gave a high level of contrast between the 12 active layers and theinterlocking electrodes. FIG. 6 offers a close-up of the interlayerelectrode and shows some of the porosity of the samples. The grain sizein the multi-layer structure was on the order of several microns. Thedevices were then mounted on an alumina substrate and leaded with asilver based epoxy for leads.

The sensor was tested in air (200 ml/min) and the resistance of thesensor element was measured as a function of changes in air composition.The testing of the devices revealed that the devices were sensitive tomoisture, to temperature, and to reducing gases. FIG. 7 shows theresponse of the device to 5% hydrogen gas. There is a decrease in theresistivity of the device on switching to hydrogen and an increase onswitching back to air. Some EMI noise is apparent in the data due to theway the meter makes the measurement. Other measurements confirmed thatthe EMI noise can be eliminated.

Further experiments confirmed that the multi-layer structure has highresponse to moisture. FIG. 8 presents the response data. The sensitivityof the sensor to water vapor in air was found to be 15 and the responsetime was less than 5 seconds.

The sensor was also responsive carbon monoxide. The sensitivity of thesensor to carbon monoxide was 0.15 and response time was over a minute.The sensor had no response to methane, which suggests that the sensor issensitive to certain reducing gases, but not all reducing gases. Thesensor showed no response to an oxidizing environment.

Example 2

Sensor Using Inductance and Capacitance Changes

Non-stoichiometric barium strontium titanate (BaSrTiO₃) multi-layerstructures of 1812 EIA standard capacitor size and NPO formulation wereground on a 40 micron diamond pad exposing the multi-layer surface andleaving an interdigital electrode with the sensing material between thefingers of the electrodes. The sensor surfaces were then polished with 9micron diamond paste. Five different sensors produced in this mannerwere tested to establish reproducibility of the performance. First eachsensor was exposed to 200 ml/min of nitrogen for one minute to establisha base line. Next, the sensor environment was changed to 200 ml/min ofanalyte containing nitrogen. Finally, the sensor environment was changedback to pure nitrogen. The capacitance of each sensor was measured as afunction of gas composition. Sensors were sensitive to hydrogen attemperatures higher than 550° C., demonstrating the device's usefulnessfor high temperature applications. The response time was about 1 minuteand the sensitivity was about 0.45. At temperatures higher than 600° C.,the sensor was found to exhibit changes in inductance as well (inductiveselectivity to hydrogen was about 20.0 at 619° C. and response time wasless than 5 seconds). This demonstrates that the sensor's capacitanceand inductance are also alternative usable signals for sensor metrology.

Example 3

Single Laminate Chemical Composition Sensor

Nano ITO powders (In₂O₃:SnO₂=1:9 wt/wt %) were prepared using chemicalprecipitation. Indium (III) chloride (Aldrich, as received) 100 g wasdissolved in 40 ml water with the addition of 70% HNO₃. Then 12.896 g of61.55% SnCl₄ solution was added to the InCl₃ solution. Constantagitation was maintained using a magnet stir bar. The indium and tinchloride solution was titrated using 14% NH₃ solution until the pH ofthe solution turned to around 10. During the titration, the clearsolution turned to milk white slurry. The slurry was filtered undervacuum, and washed first with water, then with denatured ethyl alcohol,and finally with cyclohexane. The resulting cake was dried at 100° C.for 3 hours, yielding fine white powders. The powders so produced wereball-milled for 5 hours followed by calcination at 450° C. for 30minutes, leading to yellow ITO powders. Then ITO powders werecharacterized with X-ray diffractometer which indicated that the size ofthe powders were about 13.5 nanometers.

To prepare the sensor; first a discontinuous, interdigitated goldelectrode was screen printed on alumina substrate. A single sensinglayer laminate was then deposited as follows: 100 ml 0.1 M pyrrole and0.5 M KCl solution was first prepared, and dispensed into a vial with83.3 mg of a biospecies—glucose oxidase (Sigma, G6766). Then 300 1pyrrole-biospecies solution was pipetted and galvanized under 1.0 A for2 minutes with Ag/AgCI as reference electrode, platinum as counterelectrode, and interdigitated gold electrodes prepared above as workingelectrodes. A thin layer of polypyrrole film was formed onto theinterdigitated gold electrodes under galvanization, and biospecies weregrafted on the polypyrrole nanocomposite film.

The prototype nanocomposite sensors were tested with 2000 ppmtrichloroethylene (TCE) in zero-grade synthetic air. The gas flow ratewas controlled with Mass Flow Controller at 200 ml/minute, and thesensor testing temperature was kept at room temperature (22° C.˜23° C.).The resistance spectra were recorded with Quicklog interface andsoftware from Omega Engineering, Inc. The results show that the sensorshad good sensitivity for dichloromethane and trichloroethylene. Thesensitivity ((R_(a)−R_(s))/R_(a)) was 0.15 for dichloromethane and 0.10for trichloroethylene. The response time was less than 10 seconds.

Example 4

Single Laminate Nanocomposite Sensors for Explosive Vapors

0.5 grams of Tin Oxide (SnO₂) nanopowders with a specific surface areaof 56.1 m²/g were mixed in a solution of 0.005 g/ml of poly(ethyleneglycol) binder dissolved in isopropanol and the mixture was sonicatedfor 20 minutes, producing a thick slurry The prepared slurry wasdeposited onto an alumina substrate with pre-printed electrodes, usingspin coating at a rotation speed of about 1000 rpm. Each spin coatingprocess yielded a single laminate thin film nanocomposite sensor. Toexamine the effect of catalyst dopant on the nanocomposite sensorperformance, a 2% solution of Pt/Pd catalyst in water was prepared andabout 1-2 microliter was deposited, using syringe, onto the surface ofsome sensor prototype films. The catalyst immediately spread out overthe entire surface of the film and dried in a few minutes. The sensorelements were 5 tested with explosive stimulant vapors (Sigma/Aldrich,as received) as the analyte and 200 ml/min air as the carrier gas.

Table 1 shows a summary of the sensitivities and response times for setsof deposited SnO₂ sensors to analytes as room temperature or higher.

TABLE I Sensor response to (3 ppm) analyte in 200 ml/minute ResponseTemp. Sensitivity Time Sensing Material Analyte (CO) (Rair Rg)/Rg(seconds) Nanocomposite Explosive 21  26% 65 Nanocomposite + Explosive21  40% 40 Catalyst Nanocomposite + Explosive 30 213% 25 CatalystNanocomposite + Cyclohexane 21  44% 30 Catalyst

The single laminate sensor is sensitive to explosive stimulantsignature. The sensitivity increases with the presence of a catalyticdopant and with a slight increase in device temperature. The sensor isalso sensitive to cyclohexane, another species that is commonly foundwith explosive materials, although to a somewhat different level.

Uses

From process viewpoint, the teachings of this invention can help reducethe cost of sensor, reduce the material needed, and reduce waste, whileincreasing the production rate of sensors and enhancing the efficiencyand effectiveness of the sensors.

This invention can be specifically utilized to develop and produceproducts that sense, react, trigger, or adapt to changes to environmentin general, and in the chemical composition of a fluid in particular.The invention can be generically applied to develop and produce productsthat sense, react, trigger, or adapt to changes in the environment suchas changes in the thermal state, mechanical state, magnetic state,electromagnetic state, ionic state, optical state, photonic state,chromatic state, electronic state, biological state, or nuclear state,or a combination of two or more of these. As a non-limiting example, ifthe sensing layers are prepared from thermally sensitive materialcompositions, rapid response thermal sensors can be produced. In anotherexample, if piezoelectric compositions are used in the sensing layer ina multi-laminate stack, vibration and acceleration sensors can beproduced. In yet another example, magnetic compositions can yield rapidresponse magnetic sensors and magnetoresistive sensors. If theinterlayer instead is prepared from compositions that interact withphotons, novel chromatic, luminescent, photo detectors and photoelectricdevices may be produced. With compositions interacting with nuclearradiation, sensors for detecting nuclear radiation may be produced. Inanother example, with biologically active interlayers, biomedicalsensors may be produced. With insulating interlayers, these device maybe thermally isolated or made safe and reliable. The interlayers can bemixed, as discussed before, to provide multi-functional devices andproducts. The sensing layers may be cut or left intact for specificapplications. The sensing layer may be just one layer or a multitude ofas many layers as cost-effectively desirable for the application. Theelectrode may also be one layer or a multitude of as many layers ascost-effective and necessary for the application. These sensors haveperformance characteristics desired in chemical, metallurgical,environmental, geological, petroleum, glass, ceramic, materials,semiconductor, telecommunications, electronics, electrical, automobile,aerospace and biomedical applications. Such sensors can be combined withmetrology techniques and transducers to produce “smart” products andproducts that adapt and learn from their environments.

What we claim is:
 1. A chemical sensor comprising: a laminated structurecomprising a plurality of layers of sensing material and a plurality oflayers of conductive material, wherein the plurality of layers ofsensing material alternate with the plurality of layers of conductivematerial, and wherein the sensing material of each of the plurality oflayers of sensing material is selected to exhibit a change in anelectrical property when exposed to carbon monoxide.
 2. The sensor ofclaim 1 wherein the sum total of the layers of sensing material and thelayers of conductive material of the laminated structure comprises atleast 3 layers.
 3. The sensor of claim 1 wherein at least a portion ofthe layers of the sensing material are exposed to an environmentsurrounding the sensor.
 4. The sensor of claim 3 wherein the layers ofthe sensing material interact with the environment causing an electricalresponse that is measured using the plurality of layers of conductivematerial.
 5. The chemical sensor of claim 1 wherein the sensing materialcomprises a layer that is less than 10 microns.
 6. The chemical sensorof claim 1 wherein the sensing material comprises a layer that is lessthan 1 microns.
 7. The chemical sensor of claim 1 wherein the sum totalof the layers of sensing material and the layers of conductive materialof the laminated structure comprises at least 5 alternating layers.
 8. Achemical sensor comprising: a laminated structure comprising layers ofsensing material and conductive material, wherein the sensing materialphysically interacts carbon monoxide to provide a change in the sensingmaterial's properties, and the laminated structure is arranged such thatthe change in the sensing material's properties are detectable using thelayers of conductive material.